The effect of sterol on a fatty acid biosynthetic system will be investigated using Tetrahymena as an experimental organism. It is hypothesized that an observed shift in fatty acid composition in this organism when the native compound, tetrahymanol, is replaced by ergosterol may be due to a direct effect of the added sterol on the microsomal desaturase or elongation enzymes. Sterol thus might be involved in regulation of fatty acid biosynthesis. This effect might be due to lipid-lipid interactions or to a direct association between sterol and protein in the membrane. Microsomes will be isolated from cells which contain tetrahymanol and from those containing ergosterol. The relative capacity of these fractions to incorporate radioactive precursors such as C14-palmitate into fatty acids, specifically 18:2 delta 6,11, will be investigated by incubation with the precursor followed by isolation of lipids, conversion of esterified fatty acids to methyl ester, and separation of the various unsaturated fatty acid methyl esters by argentation column chromatography. The amount of sterol present in the microsomes and the fatty acid composition of microsomal phospholipids will be determined by GLC in order to establish whether ergosterol or 18:2 delta 6,11 is accumulated in greater quantities than anticipated from control microsomes. Specific activities of various fatty acids in microsomes will be compared to show whether ergosterol causes a differential effect to synthesis. If an effect of sterol replacement on desaturase activity can be demonstrated, then isolation of the enzyme will be attempted in order to study the nature of the sterol effect. This research may provide information relevant to the function of cholesterol in humans.